1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus which performs complex vector phase correction to echo signals obtained in time sequence from each ultrasonic transducer thereof, with regard to an organism, thereby obtaining an ultrasonic tomography image with high resolution.
2. Description of the Related Art
There are ultrasonic diagnostic apparatuses which emit ultrasounds into an organism, receive the waves reflected from within the organism tissue and process the received signals so as to obtain an ultrasonic tomography image of the organism. Such an apparatus is capable of obtaining an ultrasonic tomography image without exposing the subject to X-rays as with an X-ray diagnostic apparatus which must use X-rays to obtain an X-ray photograph, and for this reason is widely used for examinations and the like in obstetrics and gynecology.
FIG. 1 illustrates an example of the block configuration of an ultrasonic diagnostic apparatuses 51 of prior art which employs the aperture synthesis method. The ultrasonic probe 52 has an array of piezoelectric transducers 53-1 through 53-5.
The signal generating circuit 54 is connected to the piezoelectric transducers 53-1 through 53-5 via transmitting amplifiers 56-1 through 56-5 connected to a transmission drive circuit 55, whereby driving signals are applied to each of the piezoelectric transducers 53-1 through 53-5, thus emitting ultrasonic pulses into the object under inspection.
Further, the ultrasonic transducers 53-1 through 53-5 each connected to a multiplexer 57 via signal lines, and one ultrasonic transducer 53-i ("i" denotes one of 1 through 5) is selected. The same ultrasonic transducer 53-i which was used for transmission is selected by the multiplexer 57 to receive the ultrasounds reflected from the object under inspection. The received ultrasounds are converted into electrical signals, and fed to a receiving amplification circuit 58. This receiving amplification circuit 58 is controlled by a STC (sensitivity time control) circuit or STC control circuit 59, and amplification to a certain amplitude is performed.
The output signals of this receiving amplification circuit 58 are input to a band pass filter 60, and following removal of unnecessary noise, are converted to digital signals by an A/D converter 61, and subsequently are stored to a wave-front memory 62 which stores wave-front data.
This operation is performed through the transducer 53-5 by switching with the multiplexer 57. After switching has been performed through the transducer 56-5 and the data has been stored in the wave-front memory 62, data corresponding with the focal point to be synthesized from the wave-front memory 62 is extracted, based on an address generated by the wave-front locus look-up table (LUT) 63, addition processing is conducted by the adding circuits 64-1 and 64-2, and squared by the squarers 65-1 and 65-2.
Since the property of wave is defined by both its amplitude and phase, the precise synthesis of a certain wave-front requires both real and imaginary components of each point of the wave-front. For this reason, both real and imaginary components are read out of the wave-front memory 62 separately, using two types of addresses generated from the wave-front locus LUT 63, namely real wave-front locus and imaginary wave-front locus. Real and imaginary components are summed and squared by the adding circuits 64-1, 64-2 and the squarers 65-1, 65-2 respectively, to produce squared real and imaginary amplitudes corresponding to the reflection from the focal point where the wave-front converges. The obtained real amplitude and imaginary amplitudes are added by the adder 66 and then output to a digital scan converter (DCS) 67.
FIG. 2 is a schematic illustration regarding the example of prior art illustrated in FIG. 1, of the state of data stored in the wave-front memory 62, the data being received echo signals quantized by the A/D converter 61 by means of quadrature sampling. Here, the blank circles indicated in FIG. 2 by a1, a2, and so forth denote sample points of quantized data, the solid circles denote sample points on the real wave-front locus, and the double circles denote sample points on the imaginary wave-front locus which of which the phase is 90.degree. offset from the real wave-front locus denoted by the solid circles.
For example, in the event that an echo signal with a center frequency of 7.5 MHz is sampled with sampling frequency of 60 MHz, the number of samples of each cycle T of the echo signal is 60/7.5=8. The time t during this sampling being converted to phase is 360.degree./8 samples=45.degree..
With the method of the prior art, data of sample points on real and imaginary wave-front locus is extracted, which is separated by respective adding circuits for real components and imaginary components and then squared, and then obtaining the synthesized output of the focal point by adding these.
This wave-front locus is a type of address information which is determined by the relative relationship regarding ultrasonic wave propagation between the focal point which is to be imaged and each transducer. By means of changing this wave-front locus, the focal point position is changed in a two-dimensional manner so as to cover the display area, and a B-mode ultrasonic tomography image which corresponds to the display area is obtained.
According to the above-described method, the echo signals are quantized by the A/D converter 61, the data obtained is extracted as complex data along wave-front locus for real component and imaginary component, synthesizing a B-mode image. According to literature ("Ultrasonic Beam Formation by Discrete Processing and the Method Thereof", Katakura et al, Japan Acoustic Institute Journal Volume 44, No. 7) which describes a beam forming method wherein signal synthesis processing is performed by using quadrature sampling technique, high time precision (i.e., phase precision) is required with synthesis according to quadrature sampling technique, in order to decrease unnecessary components in synthesized results.
However, with the above-described example of prior art, there are limitations to the conversion speed of the A/D converter 61 used in actual practice, so that the A/D converted echo signals for one cycle are of coarse quantization, i.e., 8 samples, which converted into phase precision is 45.degree.. In the event that synthesis is performed with such low phase precision, there is a problem wherein side lobes are formed by the unnecessary components, which degrade the final B-mode image, and accordingly, the B-mode image or ultrasonic tomography image with high resolution cannot be obtained.
In order to suppress such side lobes, quantizing of the echo signals to a finer degree to realize high time precision is sufficient. However, in order to realize such, a high-speed A/D converter and large capacity memory become necessary, resulting in problems such as excessive costs and complex construction.